1. Field of the Invention
This invention relates to a distance measuring device for a camera capable of automatically measuring a distance to an object and more particularly to a distance measuring device arranged to permit accurate focus detection by measuring distances to a plurality of spots located within a photo-taking image plane one after another and then by selecting an effective data from among the measured distance data thus obtained.
2. Description of the Prior Art
FIG. 4 of the accompanying drawings schematically shows the arrangement of a distance measuring device for a camera which has been proposed in U.S. patent application Ser. No. 615,784 filed May 31, 1984, now abandoned, assigned to the assignee of the present invention, which corresponds to Japanese Patent Application SHO 58-97161. According to the arrangement of the device under this previous patent application, a signal light signal is projected from a light emitting element 1; a reflection light signal resulting from this is received by means of a semiconductor position detector 2; a distance to the object is obtained from a point at which the reflection light is received on the detector 2; meanwhile, a light projecting lens 3 and a light receiving lens 4 are turned round in the direction of arrow as shown in FIG. 4 in such a way as to scan the inside of a photo-taking image plane 5 with a spot light image formed by the projected light signal; and, among the measured distance data thus obtained from a plurality of spots located within the image plane 5, a data indicative of the nearest distance is, for example, employed in adjusting the focal position of a photo-taking lens. However, in the distance measuring device arranged in this manner, it becomes hardly possible to obtain an accurate measured distance data when the projected light spot image comes to an edge portion of the object or a boundary area thereof where the reflection factor of the object or the like greatly varies during the scanning process. In other words, in that event, the incident position on the semiconductor position detector 2 of a reflection light signal coming with the projected light reflected by the object according to the principle of triangulation varies according as the object distance varies. In that event, the semiconductor position detector 2 detects the position of the center of gravity of the reflected light spot image (position of a weighted mean) as the incident position and a measured distance data is obtained on the basis of this incident position. Referring to FIG. 5, let us assume that a reflection light spot image incident on the light receiving surface 2a of the semiconductor position detector 2 becomes as indicated by a circle P when the whole of the projected light spot image is reflected by the object. Whereas, when an edge part of the object between the object and the background which does not reflect the projected light comes to be scanned with the projected light spot image, it is only, say, one half of the spot image that is reflected by the object. In that event, the reflected light spot image incident upon the surface 2a of the detector 2 becomes as represented by a hatched portion of the image P. In that event, despite of the fact that the reflection light comes from the same object to be photographed, the center-of-gravity position of the reflection light spot image shifts from a point W1 to another point W2. Accordingly, the incident position of the reflection light also changes to make an accurate measured distance data hardly obtainable. The same problem arises during a scanning process on an object having a part where its reflection factor greatly varies.
The above-stated problem sometimes arises also with the conventional devices of the kind measuring only a distance to an object located in the middle part of the image plane. This problem can be solved by always bringing the object within a distance measurement range set in the middle part of an image plane in carrying out distance measurement, though it brings about another problem in terms of operability of the device. Meanwhile, in the case of the device of the kind arranged to measure distances to a plurality of spots located within a phototaking image plane, the above-stated problem of possible erroneous distance measurement is almost unavoidably encountered during the scanning process.